1. Field of the Invention
This invention relates to a process for the nitration of benzene. More particularly, it relates to a two-stage process for the mononitration of benzene in mixed acids.
2. Description of the Prior Art
The nitration of aromatic hydrocarbons is a process of great commercial importance. Of particular importance is the mononitration of benzene.
A number of processes have been developed for nitrating aromatic hydrocarbons. Historically, the preparation of mononitrobenzene, sometimes referred to herein as nitrobenzene, has been a batch process, a typical nitrating agent being a mixed acid initially of about 32 percent HNO.sub.3, 60 percent H.sub.2 SO.sub.4 and 8 percent water. (All parts, percentages and proportions herein are by weight except where indicated otherwise.) The reaction is highly exothermic, and the process is potentially explosive. For a variety of reasons--one of which is safety-- the reaction has been controlled by slowly feeding one of the reactants to the other, and removing the reaction heat by external cooling. The initial reaction temperature is about 60.degree.C. but is allowed to rise to about 90.degree.C. near the end of the reaction period. As is well known, the final reaction mass is a two-phase system of nitrobenzene and sulfuric acid diluted with by-product water. The sulfuric acid can be separated by decantation and, for economic reasons, must be denitrated and reconcentrated, a process which involves substantial costs.
Another process for the production of nitrobenzene is disclosed in U.s. Pat. No. 2,256,999 to Castner (1941). In the Castner process, the mixed acid initially contains less than 10 percent HNO.sub.3, preferably 2 to 6 percent HNO.sub.3. The initial reaction temperature is about 90.degree.C. (obtained by mixing the H.sub.2 SO.sub.4 and HNO.sub.3), and final reaction temperature is about 110.degree.C. The small amount of HNO.sub.3 facilitates its complete reaction with the organics. This avoids the need for denitrating the acid before it is reconcentrated, and it permits conservation of the heat of reaction in the recycled acid.
In addition to batch processes with mixed acids, nitrobenzene has also been made in continuous mixed acid processes and in nitric acid processes not using sulfuric acid. One such continuous process uses two back-mixed reactors in series operating at a temperature maintained near 70.degree.C. by cooling, followed by centrifugal separation of the product from the acids, water washing, neutralization and finally distillation.
U.S. Pat. No. 2,773,911 -- Dubois et al. (1956) describes a continuous process operating at 46.degree. to 93.degree.C. for the mixed acid nitration of benzene. The reactor effluent is separated into two phases comparable to the phases occuring in batch nitrations. These phases are processed to purify the nitrobenzene and reconcentrate the spent acid.
Nitric acid alone without sulfuric acid was used in the nitration of benzene with the excess water being removed overhead as an azeotrope with benzene as reported by Othmer et al., Industrial Engineering Chemistry 34, 286 (1942). Subsequently, others also worked with azeotropic removal of water from a nitration in nitric acid alone. U.S. Pat. Nos. 2,435,314 and 2,435,544 -- Kokatnur (1948) say that the distillation avoids the need for the dehydrating effect of sulfuric acid. Although those patents show some nitrations at temperatures of 130.degree. to 150.degree.C., benzene nitration is done at temperatures up to 100.degree.C.
U.S. Pat. No. 2,739,174 -- Ross (1956) nitrates benzene or toluene at 100.degree. to 120.degree.C. with nitric acid only and uses azeotropic distillation of a bottoms stream to separate the nitrated hydrocarbon and water from the HNO.sub.3.
However, all of these processes using nitric acid alone without sulfuric acid are less than optimum in commercially important features such as reaction rate. The temperature of the reaction is limited to minimize hazards and the production of oxidation products such as dinitrophenol by the low concentration nitric acid.
An attempt to combine mixed acid nitration and azeotropic distillation of water and benzene is shown in U.S. Pat. No. 2,370,558 -- Mares (1945). The mononitration of benzene in batch and continuous processes is shown at temperatures in the range of 45.degree. to 60.degree.C., and it is stated that it could be done at from 40.degree. to 90.degree.C. Vacuum is used to aid in distilling off the azeotrope of water and benzene. Higher temperature reactions are said to be hazardous and difficult to control. Alternatively, after separation of the unwanted water, distillation can be stopped and the pressure increased to atmospheric to complete nitration at 55.degree. to 60.degree.C. in an hour in a batch process. However, reaction rates using these processes are not as rapid as would be desired for modern-day operations.
Accordingly, it would be desirable to find a process which can operate more efficiently and at a higher production rate than the prior art while not being hazardous.